Motor control may have a wide range of applications. The applications may include, among others, electrical motors for electric vehicles but also electrical motors for residential washing machines, fans, hand-held power tools, industrial motor drives, etc.
Induction or asynchronous motors are typically used in the above mentioned applications. Induction or asynchronous motors are AC motors in which a current is induced by electromagnetic induction in a rotating winding by a magnetic field generated in a static winding. Induction or asynchronous motors do not require sliding electric contacts electrically connecting the rotating winding to the static winding, thereby simplifying construction and improving reliability of the induction or asynchronous motors.
Availability of affordable, reliable power transistors (e.g., power MOSFETs and IGBTs) and modules capable to drive such induction or asynchronous motors are an important design goal in the above mentioned applications.
Typically, a gate drive circuit controls a gate of the power transistor. IGB transistors are reliable power transistors which typically have a gate charge of several microcoulombs. In order to quickly turn-on an IGB transistor with low losses a current of several amperes has to be applied to control the gate. Switching losses of IGB transistors depend on how rapidly a gate charge providing full conductivity may be fed or removed from the component gate. The gate voltage of an IGB transistor in a full conducting state is typically of the order of +15V. A gate voltage lower than +15 V may lead to excessive conduction losses and a gate voltage higher than +15 V may lead to a destruction of the IGB transistor. The IGB transistor has typically an internal gate contact resistance through which a voltage drop may be generated. Thus the gate drive circuit must provide a sufficient high output voltage to allow current to be generated through the internal gate contact resistance.
Forcing +15 V at the gate terminal of the IGB transistor may be a straightforward solution in order to guarantee a proper gate voltage to reach the full conducting state. The internal gate contact resistance and the internal gate capacitance provide an RC constant determining the speed of switching of the IGB transistor. After approximately four RC constant times, the voltage at the internal gate terminal reaches +15 V. For most applications this switching speed may not be acceptable.
EP1596496A1 provides a gate drive circuit that speed up the turning on of an IGB transistor. The gate drive circuit is provided with a specific boost capacitor added in series with the gate contact resistance. During switching off the IGB transistor the boost capacitor is charged to a voltage limited by a zener diode placed in parallel with the boost capacitor. During switching on the IGB transistor, the gate makes use of the previously stored charge in the boost capacitor and on a voltage built across the gate contact resistance to increase very rapidly the gate charge. Although EP1596496A1 provides an effective solution to speed up the turning on of an IGBT, one of the problem with this solution is that the charge across the boost capacitor may not be controlled very accurately. The only way to control the charge across the boost capacitor, according to EP1596496A1, is by correct dimensioning of the boost capacitance in relation to the gate charge of the IGBT to be controlled. Only in this case the energy stored in the boost capacitor may exhaust when the gate voltage of the IGBT has risen to +15 V.